1. Field of the Invention
The present invention relates to a communication system and, more particularly, to a system and method for determining the timing of an operation of a communication device.
2. Description of the Related Art
In recent years, in the field of optical communications, active studies have been devoted to quantum key distribution systems, which are expected to achieve high privacy levels over transmission links, and various proposals have been made.
As a basic example, a system for sharing a quantum cryptographic key between a sender and a receiver by using two bases is proposed in Bennett and Brassard, “Quantum Cryptography: Public Key Distribution and Coin Tossing,” IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India, pp. 175-179. According to this proposal, a sender transmits single photons each modulated in phase with four types of information depending on the combinations of two bases (D, R) representing quantum states and two values of random data (0, 1). A receiver receives the single photons one by one by using the bases (D, R) independently of the sender and stores received data. Thereafter, using an ordinary (classical) channel, the sender and receiver check whether or not their respective bases used in transmission and reception match. Thus, the final common secret data is determined from the received data composed only based on the matched bases.
A “plug and play” quantum key distribution system proposed by a group of the University of Geneva, Switzerland (see Rivordy, G., at al., “Automated ‘plug & play’ quantum key distribution,” Electronics Letters, Vol. 34, No. 22, pp. 2116-2117), in particular, is capable of compensating for polarization fluctuations occurring along an optical fiber transmission line and is therefore expected as a promising scheme to put polarization-sensitive quantum key distribution systems into practical use. A schematic configuration of the plug and play system is shown in FIG. 1.
Referring to FIG. 1, in the plug and play system, a receiver, which is one to receive a quantum cryptographic key, is provided with a laser LD, which generates an optical pulse P. The optical pulse P is split into two parts at an optical coupler. One of the two parts, an optical pulse P1, goes along a short path, whereas the other one, an optical pulse P2, travels along a long path. Thus, they are transmitted to a sender as double pulses.
The sender is provided with a Faraday mirror and a phase modulator A. The received optical pulses P1 and P2 are individually reflected by the faraday mirror, whereby each optical pulse is returned to the receiver with its polarization state rotated by 90 degrees. In this event, the phase modulator A phase-modulates the optical pulse P2 when the optical pulse P2 is passing through the phase modulator A, and therefore a phase-modulated optical pulse P2*a is returned to the receiver.
In the receiver, since the polarization state of each of the optical pulses P1 and P2*a received from the sender is rotated by 90 degrees, a polarization beam splitter PBS leads each received pulse into a path that is different from the path the pulse used when it was transmitted. Specifically, the received optical pulse P1 is led into the long path and phase-modulated when it is passing through a phase modulator B. Thus, a phase-modulated optical pulse P1*b arrives at the optical coupler. On the other hand, the optical pulse P2*a, phase-modulated at the sender, goes along the short path, which is different from the path at the time of transmission, and arrives at the same optical coupler. Accordingly, the optical pulse P2*a, phase-modulated at the sender, and the optical pulse P1*b, phase-modulated at the receiver, interfere with each other, and the result of this interference is detected by any one of photon detectors APD0 and APD1. Note that for the photon detectors, avalanche photodiodes are used and driven in the Geiger mode.
As described above, a single optical pulse generated at the receiver is split into two parts, and the resultant optical pulses P1 and P2 make respective round trips between the receiver and sender while individually being phase-modulated in the course. As a whole, the optical pulses P1 and P2 travel along the same optical path and then interfere with each other. Accordingly, delay variations attributable to the optical fiber transmission line are compensated for, and the result of interference observed by the photon detector APD0 or APD1 depends only on the difference between the amount of phase modulation at the sender and the amount of phase modulation at the receiver.
The plug and play system having such a configuration requires synchronization as cited below.
(1) In the sender, it is necessary to apply a voltage corresponding to the amount of phase modulation to the phase modulator A synchronously with the timing when the optical pulse P2 transmitted from the receiver is passing through the phase modulator A.
(2) In the receiver, it is necessary to apply a voltage corresponding to the amount of phase modulation to the phase modulator B synchronously with the timing when the optical pulse P1 reflected from the sender is passing through the phase modulator B.
(3) Further in the receiver, it is necessary to apply a bias to the photon detectors APD0 and APD1 synchronously with the timing of the incidence of the returned optical pulse (super-high sensitive reception in the gated Geiger mode).
As described above, for a quantum key distribution system to stably generate a quantum cryptographic key by achieving high interference in practice, it is indispensable to perform timing control such that each of the sender-side phase modulator A, receiver-side phase modulator B, and photon detectors APD is driven in synchronization with the timing of the arrival of an optical pulse.
This would not be particularly problematic to ordinary or classical optical communications. However, in a quantum key distribution system, the number of photons per pulse is extremely small: one photon per pulse at most. Therefore, most of data transmitted from a sender is lost due to losses attributable to a transmission line, and little data arrives at a receiver. In addition to this, photons cannot be retimed because the values of arriving data are probabilistically determined. Therefore, employed is a scheme in which, apart from a quantum signal for key distribution, a clock signal, which provides timing information, is exchanged between the sender and receiver to adjust timing.
However, even in the case where the clock signal is exchanged through an optical transmission line, the extension and contraction of the optical transmission line must be taken into account. For example, in the case of an optical fiber over a distance of 20 km, if the environmental temperature rises by 10° C., an extension of 3.2 m occurs. This amount of extension is equivalent to one bit in the case of a system clock of 62.5 MHz.
A deviation in timing between a quantum signal and a synchronization signal due to the extension/contraction of an optical transmission line can be avoided by wavelength-multiplexing the quantum signal and clock signal to allow them to propagate along the same optical transmission line, using the wavelength division multiplexing (WDM) technology.
For example, Japanese Patent Application Unexamined Publication No. H08-505019 proposes a method of calibrating a system as well as bit synchronization by utilizing a classical channel. According to this method, a quantum channel and a classical channel are set on the same transmission line, and the classical channel is used to provide clock synchronization for the quantum channel, whose optical power is weak.
However, when the quantum signal and clock signal are transmitted by wavelength division multiplexing transmission, a propagation delay difference arises because the different-wavelength channels have different group velocity dispersions (GVD). Accordingly, a deviation in timing still occurs between the quantum signal and clock signal. Such a propagation delay difference between channels is a grave problem to solve, for a system that requires synchronization between different-channel signals.
Additionally, in a system that transmits information by utilizing phase modulation, such as a quantum key distribution system as described above, it cannot be determined whether or not the timing of driving a phase modulator in a sender is right, without referring to the result of detection obtained at a receiver. Therefore, even if the phase modulator is driven in accordance with a clock compensated for a propagation delay difference, it cannot be determined whether or not the result of that compensation is appropriate, without checking the result of reception at the receiver.